EP0465362B1 - Procédé d'estimation et d'étalonnage de la lumination reçue par un film radiographique - Google Patents

Procédé d'estimation et d'étalonnage de la lumination reçue par un film radiographique Download PDF

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Publication number
EP0465362B1
EP0465362B1 EP91401837A EP91401837A EP0465362B1 EP 0465362 B1 EP0465362 B1 EP 0465362B1 EP 91401837 A EP91401837 A EP 91401837A EP 91401837 A EP91401837 A EP 91401837A EP 0465362 B1 EP0465362 B1 EP 0465362B1
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EP
European Patent Office
Prior art keywords
film
refo
cnrt
exposure
lumination
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Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
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EP91401837A
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German (de)
English (en)
French (fr)
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EP0465362A1 (fr
Inventor
Robert Heidsieck
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General Electric CGR SA
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General Electric CGR SA
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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/58—Testing, adjusting or calibrating thereof
    • A61B6/582—Calibration
    • A61B6/583—Calibration using calibration phantoms
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05G—X-RAY TECHNIQUE
    • H05G1/00—X-ray apparatus involving X-ray tubes; Circuits therefor
    • H05G1/08—Electrical details
    • H05G1/26—Measuring, controlling or protecting
    • H05G1/30—Controlling
    • H05G1/46—Combined control of different quantities, e.g. exposure time as well as voltage or current

Definitions

  • the invention relates to radiology systems using a radiographic film and, more particularly in such systems, a method which makes it possible to calibrate the lumination received by the radiographic film in order to obtain a given blackening of said film.
  • a radiology system essentially comprises an X-ray tube and a detector of such radiation between which the object to be examined is interposed, such as a part of the body of a patient.
  • the image receptor which is, for example, a film-screen pair, provides an image of the object after an appropriate exposure time and development of the film. So that the image of the object can be exploited in the best conditions, it is necessary that the various points which constitute this image present between them a sufficient contrast, that is to say that the blackening of the radiographic film is correct, and that from one radiography to the next, despite the differences in opacity that the radiographed object can present.
  • the blackening of the film is linked to the amount of energy of the radiation incident on the film-screen pair, that is to say the product of the intensity of the radiation to which the radiographic film is subjected, or dose rate "film ", by the time during which the film is exposed to this radiation. Consequently, in order to obtain a constant blackening of the film from one radiography to the next, it is known (see US Pat. No. 4,748,649) to measure, during the examination, the energy incident on the film by means of a detection cell, generally placed before the receiver, which is sensitive to X-rays and provides a current proportional to the "film” dose rate. This current is integrated, from the start of the installation, in an integrating circuit which provides an increasing value during the installation.
  • This increasing value is compared during the exposure time to a fixed reference value, previously established as a function of the characteristics of the film.
  • the end of the exposure time is determined by the instant at which the comparison indicates that the value representative of the energy incident on the film is equal to the set value.
  • the blackening of the film depends on the quality of the spectrum.
  • the response of the screen depends on the energy distribution of the spectrum of the radiation received, which means that it is sensitive to the spectrum hardening and to the change in voltage of the X-ray tube.
  • the object of the present invention is therefore to implement a method which makes it possible to estimate and calibrate the lumination received by an X-ray film using measurements made by the detector placed behind the X-ray film.
  • the invention relates to a method for calibrating the lumination received by an X-ray film in order to obtain a given optical density according to claim 1.
  • the CNRT coefficient (t o ) can be determined in different ways.
  • a radiology system to which the method for automatically determining the exposure time of an object to be radiographed 13 according to the invention applies comprises a source 11 of X-ray radiation such as an X-ray tube which provides a beam 14 of rays X illuminating this object 13 and an image receptor 17, such as a film-screen pair, which is placed so as to receive the X-rays having passed through said object and which provides, after an appropriate exposure time S and development of the film, an image of the object 13.
  • a source 11 of X-ray radiation such as an X-ray tube which provides a beam 14 of rays X illuminating this object 13 and an image receptor 17, such as a film-screen pair, which is placed so as to receive the X-rays having passed through said object and which provides, after an appropriate exposure time S and development of the film, an image of the object 13.
  • the system further comprises a detection cell 12 which is placed behind the image receiver 17 in the case of an X-ray film with an intensifying screen.
  • This cell can be placed in front of the receiver in the case of a film without a reinforcing screen.
  • the detection cell 12 makes it possible to convert a physical quantity characteristic of the X-ray having passed through the object and the image receptor, such as KERMA or the energy fluence, into a measurement signal L, for example of the electrical type.
  • the signal L supplied by the detection cell 12, is applied to a circuit 16 which integrates the electrical signal for the duration S of the exposure.
  • the signal M which results from the integration, is a measurement of the radiation having passed through the object 13 during the duration S of the exposure.
  • the X-ray source 11 is associated with a supply device 15 which supplies a variable high supply voltage V m to the X-ray tube and which comprises a device for measuring the anode current I of said tube.
  • the supply device 15 and the X-ray tube comprise means for starting the emission of X-rays at a precise instant and stopping it after a variable duration S which is determined as a function of the signal M supplied by the circuit 16 and the values of I, S and V m and, more precisely, of the ratio M / I x S which is called efficiency D and which is calculated by the device 18.
  • the values of efficiency D are processed by a computer or microprocessor 19.
  • the first operation consists in performing a calibration of the radiology system of FIG. 1 which results in a function for estimating the lumination seen by the radiographic film.
  • the method for estimating the lumination received by an x-ray film is based on calibrations leading to the definition of a function proportional to the photon speed on the film, called film flow, and on a calibration making it possible to make the link between the film-flow function and the lumination received by the film under fixed reference conditions and resulting in a given blackening of the film. This latter calibration will be described in greater detail in the following description.
  • the calibrations used to define the film flow function are derived from a calibration process described in patent application No. 89 07686 (corresponding to EP-A-0402244) filed on June 9, 1989 and entitled "PROCESS FOR CALIBRATING A RADIOLOGICAL SYSTEM AND MEASURING THE EQUIVALENT THICKNESS OF AN OBJECT".
  • This process consists in measuring the efficiency D of the cell for each standard at the supply voltages V m chosen. More precisely, with a first standard of thickness E1, a performance measurement D 1m is carried out for each value V m constituting a determined set. These values D 1m as a function of the voltage V m can be plotted on a diagram to obtain the points 21 ′ of FIG. 2.
  • the yield measurements D are carried out for another standard of thickness E2 and we obtain the values D 2m corresponding to the points 22 ′ of FIG. 2 and so on to obtain the other series of points 23 ′, 24 ′ and 25 ′ corresponding respectively to the yields D 3m D 4m and D 5m and to the thicknesses E3, E4 and E5.
  • the parameters of the analytical model can be adjusted using conventional estimation tools such as the method of minimizing the quadratic error.
  • a pair of values (E p , V m ) corresponds to a performance measurement D, which makes it possible to determine E p as a function of V m and D.
  • a measurement of efficiency D which is carried out with a given supply voltage V m makes it possible to determine an equivalent thickness expressed in the units used for E p .
  • This function D f does not take into account the modification of the X-ray spectrum due to the additional filtration between the intensifying screen and the detection cell 12 which comes, for example, from the exit face of the cassette containing the film couple -screen.
  • E p in equation (8) is replaced by (E p - filter sup) where filter sup is the thickness equivalent to the radiographed object corresponding to this filtration.
  • This equivalent thickness is obtained by placing, for example, in the beam 14 an object equivalent to this filtration and by using the calibrated function determining the equivalent thickness g ′ or g ⁇ according to.
  • D f x I xt is proportional to the energy absorbed in the intensifying screen during a time t and for an anode current I
  • D f xI is proportional to the photon flow incident on the film and is expressed in them units of measurement of the signal from the detector cell 12. This proportionality relationship is all the more verified when the number of light photons emitted by the intensifying screen is itself proportional to the energy absorbed. If the number of light photons emitted by the screen responds to another law as a function of the energy absorbed, this other law must be applied to D f xI to obtain the "film flow".
  • a final calibration consists in connecting the electrical functions previously described to a value of the blackening of the film, that is to say an optical density, which it is desired to obtain at the end of the installation.
  • a value of the blackening of the film that is to say an optical density, which it is desired to obtain at the end of the installation.
  • the choice of this value is made by the practitioner according to the film-screen pair, the type of diagnosis, the part of the patient's body to be examined and his habits of examination of radiographs. This choice makes it possible to determine the reference lumination, denoted L ref , that is to say the lumination which the film must receive, under fixed reference conditions, to arrive at such blackening.
  • lumination the product of the quantity of light received, for example the illumination EC of the sensitive surface, by the duration of the exposure or exposure.
  • the operation (e3) consists in measuring the integrated value D provided by the device 18 a certain time t ′ after the start of the pose, knowing that the integrator 16 has been reset to zero, that is to say at the start of the pose, either after the last measurement.
  • the integration time t ′ corresponds, as the case may be, to the time elapsed since the start of the pose or to the time elapsed since the last measurement.
  • the operation (e4) is performed by the microprocessor 19 from the first calibration of the radiology system as described above: it is governed by equation (7); one then obtains a value E1 of the equivalent thickness.
  • the operation (e5) consists in calculating the yield of the film D f1 corresponding to the thickness E1 using the function defined by equation (8), which makes it possible to take into account, in particular, the influence of the receiver screen. This operation has been briefly described above.
  • the operation (e10) consists in making a choice: either stop the exposure, or continue it according to the value of the mAs remaining to be debited or the exposure time remaining to run, or recalculate the estimate of the forecast value of the time of end of installation.
  • This additional test consists in not modifying the value of the estimate mAs ra in the case where t rc is less than a value t o . Then the end of the pose ends in open loop by continuing only the end of pose operations, that is to say the decrementation of the number of mA.s debited and stop of the pose when this number becomes less or equal to zero.
  • the operations for estimating the time remaining and that for cutting off the exposure can be decoupled in order to further refine the accuracy of the exhibitor.
  • the process breaks down as follows: a T.E. task intended to estimate the mA.s remaining to be debited before the end of the pose and a T.C. task for cutting the pose. These are two independent tasks that take place in parallel.
  • the task of estimating TE of the mA.s remaining to be debited consists of operations (e3) to (e8) to which is added an operation (e′9) of converting mA.s into a signal in the units of the cell 12 such as:
  • THIS target mAs ra x D vs
  • This estimation task TE is renewed periodically during the pose, for example at instants t1, t2, ... t n which are measurement instants separated by a duration which is at least equal to the computation time t c .
  • the target value CE target of the cut-off task TC is updated. This update must take into account the signal received by the detector cell 12 between the instant of measurement at the start of the operation (e3) and the instant of updating the target CE value at the end of the TC operation
  • the TC pose cut-off task is a task consisting in decrementing a given or target value for cell 12 as a function of the signal actually received by this cell. This task cuts the pose as soon as the target CE value becomes less than or equal to Val o , equal to zero for example.
  • CNRD is the function representing the effect of non-reciprocity expressed as a function of the photon speed on the film.
  • the CNRD function is obtained by a calibration process which is described in the patent application (EP-A-0465361) filed today and entitled: "METHOD FOR DETERMINING THE FUNCTION REPRESENTING THE EFFECT OF NON-RECIPROCITY OF A FILM RADIOGRAPHIC ".
  • this calibration process firstly consists in determining the coefficients of non-reciprocity of the film as a function of the exposure time t i and denoted CNRT (t i ).
  • This CNRT function is determined experimentally and can be represented by an analytical function.
  • This curve shows that the lumination required for reach the desired optical density increases with the exposure time.
  • the ratio between the energies for the two exposure times of 50 ms and 6.5 s is of the order of 1.6.
  • CNRD (d i ) A ′ o + A′1 log 1 / d + A′2 [log 1 / d] 2
  • L ref is the lumination that the film receives under fixed and known radiological conditions when the film reaches a given darkening and that the effect of non-reciprocity is corrected.
  • the reference lumination depends on the optical density which it is desired to obtain on the film.
  • the first step is to make a sensitogram of the type of film used, then it is necessary to take a picture under determined radiological conditions with a standard of known thickness.
  • the DO refo reference optical density makes it possible to calculate the illumination step corresponding to DO refo on the sensitometric curve of the film used (FIG. 5), this curve having been plotted using a sensitograph and a densitometer . This allows to take into account the characteristics of the developing machine that is used.
  • the curve is recorded, for example, as a function in the microprocessor 19 (fig.1).
  • the optical density measured DO m makes it possible to calculate the measurement step Ech m which is the value of the illumination step corresponding to DO m on the sensitometric curve (fig. 5).
  • the sensitometric constant K corresponds to the scale used for the lighting steps.
  • L ref depends on t o across L film by equations (23) and (25).
  • the value L ref is sensitive to the effects of non-reciprocity of the film.
  • the latter defines the configuration whose characteristics are transmitted to the microprocessor (19) so that the latter uses the corresponding models.
  • the method according to the invention has been described in its application to a receiver 17 of the film-screen pair type. It can also be implemented in the case of a receiver 17 comprising only one film sensitive to X-radiation. With such a film,
  • the sensitograph can, in this case, be of the X emission type.
  • the detection cell 12 can be placed either behind the receiver 17 as in the case of the film-screen type receiver, or in front of the receiver 17 if the energy radiation allows it.

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EP91401837A 1990-07-06 1991-07-03 Procédé d'estimation et d'étalonnage de la lumination reçue par un film radiographique Expired - Lifetime EP0465362B1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FR9008627A FR2664397B1 (fr) 1990-07-06 1990-07-06 Procede d'estimation et d'etalonnage de la lumination recue par un film radiographique.
FR9008627 1990-07-06

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EP0465362A1 EP0465362A1 (fr) 1992-01-08
EP0465362B1 true EP0465362B1 (fr) 1995-01-11

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EP91401837A Expired - Lifetime EP0465362B1 (fr) 1990-07-06 1991-07-03 Procédé d'estimation et d'étalonnage de la lumination reçue par un film radiographique

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US (1) US5166969A (fi)
EP (1) EP0465362B1 (fi)
DE (1) DE69106608T2 (fi)
FI (1) FI117785B (fi)
FR (1) FR2664397B1 (fi)

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Publication number Priority date Publication date Assignee Title
DE9219111U1 (de) * 1992-07-10 1998-01-22 Siemens Ag Röntgenbelichtungsautomat für die Mammographie
FR2703237B1 (fr) * 1993-03-29 1995-05-19 Ge Medical Syst Sa Mammographe équipé d'un dispositif de prises en vues stéréotaxiques à détecteur numérique et procédé d'utilisation d'un tel mammographe .
GB9904692D0 (en) 1999-03-01 1999-04-21 Isis Innovation X-ray image processing
GB0028491D0 (en) * 2000-11-22 2001-01-10 Isis Innovation Detection of features in images
US20090080597A1 (en) * 2007-09-26 2009-03-26 Samit Kumar Basu System and method for performing material decomposition using an overdetermined system of equations
US7597476B2 (en) * 2007-11-07 2009-10-06 Dornier Medtech Systems Gmbh Apparatus and method for determining air-kerma rate

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NL7710052A (nl) * 1977-09-14 1979-03-16 Philips Nv Inrichting voor computer-tomografie.
FI79241C (fi) * 1985-08-29 1989-12-11 Orion Yhtymae Oy Foerfarande och anordning foer reglering av roentgenstraolningen vid en roentgenanordning, speciellt en mammografianordning.
US4748649A (en) * 1986-08-04 1988-05-31 Picker International, Inc. Phototiming control method and apparatus
US5068788A (en) * 1988-11-29 1991-11-26 Columbia Scientific Inc. Quantitative computed tomography system
US4980905A (en) * 1989-02-16 1990-12-25 General Electric Company X-ray imaging apparatus dose calibration method

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FI913244A (fi) 1992-01-07
EP0465362A1 (fr) 1992-01-08
FI913244A0 (fi) 1991-07-04
FR2664397A1 (fr) 1992-01-10
FR2664397B1 (fr) 1992-09-11
FI117785B (fi) 2007-02-15
DE69106608D1 (de) 1995-02-23
DE69106608T2 (de) 1995-05-24
US5166969A (en) 1992-11-24

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